First of all, a bow cannot be used as an improvised melee weapon as suggested upthread. PHB p 215, "You can't use a ranged weapon as a melee weapon."
Next, I think by the letter of the rules, you can't use Twin Strike when you are unarmed, because the unarmed "weapon" does not have the off hand property. So you can't be considered to be using(*) two weapons to satisfy the requirement of the power. If the unarmed weapon had the off hand property it would be allowed.
In a recent WotC article on making 4e conversions of 3e characters without official classes, the author (Mearls?) recommended the monk conversion, built on ranger, get a feature granting (among other things) the off hand property to his unarmed attack, pretty much expressly to allow use of unarmed in conjunction with Twin Strike, aka Flurry of Blows.)
In the end, I think allowing that ranger to attack with two kicks or a kick and a punch, would pretty much be ok, as would be letting him use the bow as an improvised weapon. It's a subpar option that wouldn't break anything, and might be good for some drama and fun in many games.
[D]A[/D]
* - I say "using" instead of "wield" or "hold". I think those terms are a distinction without a difference as far as the rules go. I read all three as equivalent terms.
This is what the off-hand property grants:
PHB 217
Off-hand: An off-hand weapon is light enough that you can hold it and attack effectively with it while holding a weapon in your main hand.
PHB 215 (end of Weapon Categories)
Some one-handed weapons are light enough for you to use in your off hand while holding another one-handed weapon in your other hand.
And "wielding":
PHB 56
Weapon: Many martial powers... can be used only if you're wielding a weapon. (You can use an unarmed attack as your weapon.)
Wield = Use = Hold = Able to attack with . It's all the same as far as the rules. (Yes, you can make a distinction that you cannot "hold" an unarmed attack, but that is a semantic distinction not relevant to the rules. I could make a case that I can hold a greataxe in one hand, clumsily, and not be able to attack with it... but would I be "holding" it to satisfy the Two-Weapon feats?)